The present invention relates to an optical system for a video camera employing a TTL (Through The Lens) automatic focusing device. In a conventional video camera with a TTL automatic focusing device, a light beam passing through a main image pickup lens is divided into two parts by a beam splitter. One of the divided light beams is applied to an image pickup element while the other is applied to an automatic focusing sensor. Heretofore, the beam splitter usually employed has been a half-silvered mirror. In general, in order to increase the accuracy of automatic focusing (i.e., the sensitivity to defocusing) it has been the practice to increase the focal length of a relay lens system for the automatic focusing sensor with respect to the relay lens system of the main image pickup lens. In the conventional automatic focusing device, a mirror is disposed on the optical path, after the position where beam splitting is carried out, to deflect the light beam, thereby to ensure the optical path length of the light beam needed for automatic focusing control. Accordingly, the conventional automatic focusing device is necessarily bulky and costly both in terms of components and maintenance.
In order to eliminate the above-described difficulties, the present applicant has proposed a TTL automatic focusing distance-measuring optical system in which a light beam used for automatic focusing provided by means of a half-silvered mirror type beam splitter is reflected by a mirror so as to cause it to pass through the beam splitter again to thereby reduce the optical path length, achieving both miniaturization and a reduction in the number of components.
FIGS. 1A and 1B are a side view and a front view, respectively, showing this conventional optical system for a TTL automatic focusing video camera. In the case of a zoom lens, a light beam passed through a lens group 1 passes through a relay lens 2, and is then split into an image pickup beam and an automatic focusing control beam by a glass block 3 having a half-silvered surface 3a. The image pickup beam passes through a relay lens 4 to reach an image pickup element 5. The automatic focusing control beam is applied to an aberration correction lens 6, used for the automatic focus sensor, whose rear surface 6a has a totally reflecting coating. The automatic focusing control beam is reflected and converged by the rear surface 6a, and is then applied through the glass block 3 to an automatic focusing sensor 7. In FIG. 1A, reference numeral 8 designates an aperture.
In the above-described conventional optical system, the light beam applied to the automatic focusing sensor 7 passes through the glass block 3 twice so that the length of the optical path is increased without increasing the dimensions of the optical system. However, in this case, because the light beam passes through the half-silvered surface 3a twice, the quantity of light incident upon the automatic focus sensor 7 is unavoidably decreased. The part of the light beam which reaches the image pickup element 5 is reflected by the glass surface of the element 5. This reflected light beam adversely affects the automatic focusing control when applied to the automatic focus sensor 7.
This optical system provides significant merits. However, it is still disadvantageous in that, as the light beam is passed through the half-silvered mirror type beam splitter twice, the quantity of light incident upon the automatic focusing sensor is decreased. Also, the automatic focusing sensor is adversely affected by stray light which is formed when the light beam is reflected by the glass surface of the image pickup element in the video camera.